With "grub2: update to newer grub2 2.06 version" a regression slipped in
which makes the ISO not bootable on real hardware. The commit reverts to
the previous behaviour, to load the GRUB2 modules after boot later one by
one in the ISO format case.
Fixes#4647
Issue #2778
The "create_grub2" script functionality is moved to the g2fg contrib sources
and also the GRUB2 head image (tool/grub2-head.img). The head image is now
partitioned as GPT and bootable in BIOS legacy mode.